RAD 354 Chapt . 13 Intensifying Screens

1. RAD 354 Chapt. 13 Intensifying Screens • Physical purpose: to convert x-ray photons into light photons (done at the phosphor layer). The RESULT does lower patient dose.

2. Most in use – if not ALL – are “rare earth” • Rare earth crystals include (but are NOT limited to): • Gadolinium • Lanthanum • Yttrium

3. Other Intensifying Crystals Used • Barium lead sulfate (very early phosphor used) • Calcium Tungstate

4. Desired Physical Properties of Crystals • High atomic number = high absorption (DETECTIVE QUANTUM EFFICIENCY {DQE}) • Phosphor should emit a LARGE # of light photons for EACH x-ray photon – CONVERSION EFFICIENCY (CE) • Color of light should match the color light the film is sensitive to – SPECTRAL MATCHING • ZERO afterglow (“lag”)

5. Important Screen Terms • Luminescence – process of giving off light when stimulated • Fluorescence – giving off light ONLY when stimulated • Phosphorescence – continuing to give off light after stimualation • Intensification factor – amount of radiation reduction WITH screens vs NO screens

6. Screen Speed • Can be judged by intensification factor (IF) • Increasing speed INCREASES noise • Increasing speed REDUCES spatial rresolution • Increasing speed INCREASES quantum mottle (line-pair test pattern device is used to measure this)

7. Tech CONTROLABLE Screen Items • Screen attributes the tech can control: • Radiation quality (kVp, grid/no grid, filters, etc.) • Image processing and temperature • Care of and cleaning of screens

8. Cassette Construction • Rigid, light proof protective housing for the film and screens • Felt/rubber/sponge “compression” layer to assure good film-screen contact • K-edge of crystals determines light spectrum

9. Screen Cleaning • Compare/contrast screen cleaning solutions (home made vs commercially produced) • Cotton balls vs 4 X 4’s

10. Screen – Film Contact Test • Wire mesh test for screen-film contact and proper resolution/visibility of detail

11. RAD 254 Chapt. 14 Control of Scatter • Break down into: Those that reduce patient dose and those that are geometrical in nature and those not

12. 3 (primary) factors affecting scatter • Increased kVp • Increased field size • Increased patient thickness

13. Spatial Resolution & Contrast Resolution • Spatial resolution may be thought of as geometric in nature (F.S. size, emission spectrum, OID, SID – dealing with geometric image formation • Contrast resolution – driven by scatter and other sources of “noise”

14. Scatter • INCREASED filed sizes = MORE scatter – collimation is the MOST readily available and EASIEST thing to lower the amount of scatter • Patient thickness also INCREASES scatter – compression may be used to help avoid this (IVP’s and mammos are examples where compression may be used)

15. Beam restricting devices limit the radiation to the patient • Aperaturediaphram (size and resultant field size are a DIRECT proportion – draw the damn picture and figure the problem) • Cones and cylinders – GREAT for absorbing scatter, but are circular shaped = great for improving contrast and removing scatter, BUT required MUCH MORE mAs as a result

16. Variable AperatureDiaphram • Mandated in 1974 by the Food and Drug Administration (mandate later removed) • Positive Beam Limitation Device (PBL’s) • Automatically collimate to the size of the cassette/receptor in the bucky and CANNOT be a BIGGER size than the cassette/receptor

17. Filtration • Filtration also will DECREASE the low energy rays and LIMIT patient dose and some scatter

18. The Grid • Only “FORWARD” scatter is of any benefit to the radiographic image – ALL other scatter degrades the image!

19. Scatter = LOWER Contrast • Using a grid (alternating strips of fine leaded strips with alternating radiolucent interspace material) can effectively reduce the amount of ANGLED scatter from reaching the cassette/receptor

20. Grid Terms • Grid ratio = height of the lead lines divided by the interspace width • Grid frequency/lines per inch = the MORE lines per inch, the more clean up • Grid clean up = scatter w/o a grid vs scatter reaching the film/receptor with a grid AKA “Contrast Improvement Factor” • Grid function = improved image contrast

21. Bucky Factor • Refers to the AMOUNT of radiation to the patient with a grid vs W/O a grid • The HIGHER the grid ratio, the HIGHER the “bucky factor” • The HIGHER the kVp, the HIGHER the “bucky factor” • Grid WEIGHT refers to how HEAVY the grid is – duhhhh- the MORE lead the heavier it is

22. Grid Types • Parallel • Crossed (cross hatch) • Focused • Focused crossed

23. Grid Problems • Grid cut-off = short SID’s result in the vertical, parallel strips absorbing the “diverging” beam at the OUTER margins of the grid/film/receptor; MOST pronounced at SHORT SID’s • Most grid problems are positioning related • Uneven grid/off level grid • Off centered (lateral decentering) • Off focus grid • Upside down, focused grid

24. Focused Grid Misalignment • Off level = grid cutoff across image; underexposed image (light OD) • Off Center = ditto • Off focus = CR centered to one side of the other of a focused grid • Upside down grid = SEVER grid cut-off (NO density/OD) at BOTH sides of the image

25. Grid Ratio Selection • 8:1 grid is the MOST widely used • 5:1 grid is the most PORTABLE use grid ration • Grid ratio is kVp driven • Higher kVp’s warrant HIGHER grid ratios • Higher grid ratios = HIGHER patient dose (more radiation needed to produce an image) • As kVp increases pat MAXIUM OPTIMUM kVp, patient dose INCREASES

26. mAs – Grid Considerations • AS grid ratio INCREASES, so must mAs • 5:1 = 2 X mAs • 8:1 = 4 X mAs • 12:1 = 5 X mAs • 16:1 = 6 X mAs

27. Air Gap Technique • By allowing the scatter radiation to “diffuse” in the atmosphere AFTER the patient but BEFORE the cassette/receptor, the image has HIGHER contrast, as the scatter diffuses and does NOT reach the receptor • C-spine is a good example of this

28. RAD 354 Chap. 15 Radiographic Technique • Four PRIMARY exposure factors: • kVp • mA • Time • distance

29. In the next 5 minutes • Write down “bullets” about what happens when on RAISES kVp

30. Memory “jerk” for grids • Write the following: • 5 2 • 8 4 • 12 5 • 16 6

31. Now What??? • 5:1 = 2X mAs • 8:1 = 4 X mAs • 12:1 = 5 X mAs • 16:1 = 6 X mAs

32. kVp • Beam Qualtiy • Primarily responsible for quality, BUT INCREASES in kVp also make x-ray production SLIGHT more productive • Penatration • Beam intensity • HVL • Biggest exposure factor affecting CONTRAST

33. mA • DIRECTLY responsible for AMOUNT of radiation produced (Quantity). As mAs is doubled, so is the number of photons produced and so is PATIENT DOSE • mA stations are responsible for focal spot size selection

34. Time • Exposure times should be practical and short enough to stop patient motion, but the shortest times also result in the most radiation output per unit of time – thus MORE wear and tear on the x-ray tube • mAs = time X mA • mAs is only measured by tube current • Responsible for Optical Density (OD)

35. Distance (SID) • The most “forgotten” exposure factor, but perhaps the most important • Inverse Square Law • Primarily effects Optical Density (OD) • NO effect on quality • Other distance related terms: • FFD, FOD, OFD, FRD, ORD, SSD • Other geometric factors (F.S. size, pt. size, part orientation to CR and receptor

36. FiltrationkVp driven • Inherent (.5 mm al equiv) • Added (2.0 which may also include some filtration from localizer light apparatus, etc.) in a 70-80 kVp unit • Total filtration : inherent + added (2.5 mm al equivalent)

37. Generators • Half wave (120 cycles/sec = 60 impulses per second) – 100% ripple • “self rectified” is also half wave where the X-RAY TUBE is the DIODE • Full wave rectification (120 cycles per second = 120 impulses per second) – 1--% ripple • 3 phase, 6 pulse = 14% ripple (33% more radiation per exposure over full wave) • 3phase, 12 pulse = 4% ripple (40% more per exposure over full wave • Hi frequency = <1% ripple